Many photocopying processes involve the electrostatic charging of a sheet or roll of paper and the subsequent exposure of the charged surface to a light image of the document which is to be copied. The application of the electrostatic charge to the paper should be as uniform as possible to avoid differences in sensitivity over the surface of the paper.
Hitherto a charging device has typically comprised one or more lengths of wire which are stretched between insulating supports and are charged to a high potential from a high tension source and adjacent to which the paper is moved. Although a non-uniform pattern of charge would result from a static exposure of a sheet of paper to the charging device a relatively uniform charge can be produced over the surface of the paper by moving the paper relative to the charging device.
In addition a sufficient level of charge is obtained by using a charging device at a very much higher potential than in theory is necessary so as to ensure that the whole surface of the paper receives a charge which exceeds a minimum potential.
Known devices therefore require very high potential sources and appropriate traction means for moving the paper uniformly past the charging device and are not particularly suited to photocopying systems in which the power supply requirements are limited and/or in which the speed of movement of the paper past the charging device is not necessarily well controlled or uniform and in which the source of high tension does not necessarily produce a continuous charge but produces a series of pulses as in the case of a power supply incorporating a piezo-electric crystal which is squeezed so as to produce a high voltage across the crystal which can then be dissipated through the charging device onto the paper.